An Auxiliary Power Unit (APU) provides auxiliary power to the various accessories in an aircraft (ex. lights, etc.) and is conventionally suspended by a plurality of rigid linkages which attach between the fuselage ceiling and the APU. In prior art systems, vibration transmission into the linkages is minimized by directly attaching the rigid linkages to vibration isolators located at, and secured to, the APU (see FIG. 1a). Multiple links generally attach to multiple isolators. For example, in one prior art system, seven links connect into only three isolators. Two links connect to the first and second isolators with three links attached to the third. Notably, in these systems the isolators are placed at the hot location of the engine, thus, the isolators must be manufactured from materials that can withstand very elevated temperatures. Moreover, prior art linkage members include mechanical rod ends having ball and race members to allow pivotal motions, but such mechanical rod end bearings are subject to wear and fretting and can exhibit rattle spaces which can transmit undesirable vibration into the fuselage structure.
One example of a Prior Art APU suspension system is shown in FIG. 1a and suspends an APU 22 (only a portion is shown) from multiple rigid linkage assemblies 10 (only two of seven links shown) which attach at multiple spaced devises 11a located near the ceiling of the fuselage structure 24. Multiple rigid metal spherical rod end members 12a, 12b including right and left threads, respectively, are included at either end of each linkage assembly 10. Such rigid metal spherical rod ends include a steel housing and a spherical metal ball and may include a low friction race, such as manufactured from rigid low friction material such as phosfur bronze or Nyloy. Likewise, the linkages 10 attach to multiple isolators 13 (only one of three shown) located on the APU 22 by way of metal spherical rod ends 12b attached in clevis members 11b extending from the isolator housing 18. These isolators 13 include a housing 18 having a pocket formed therein for receiving the bonded inner member 14 manufactured from a fluorosilicone material as best shown in cross section in FIG. 1b.
The bonded inner member 14 includes a cone bolt 16 including a tapered section 17a which is received in a like tapered section 17b formed into an engine attachment bracket 15. As shown in FIG. 1b, the bonded inner member l4 includes upper and lower elastomer pads 19a, 19b spaced above and below a flange 19c formed on the rigid inner member 19d. These prior art elastomer isolators 13 tend to be bulky, heavy and expensive, thereby leading to a system that is resultantly complex, heavy and expensive. Moreover, should any one of the linkages 10 have a resonance condition within the APU's operating frequency range, it is difficult to address it without appreciably affecting the remainder of the system. In other words, the resonance can only be addressed by changing the stiffness/damping of elastomer pads 19a, 19b. Thus, addressing the resonance may change a stiffness which appreciably affects the dynamic properties of the overall system. Furthermore, the isolators of such prior art systems are exposed to intense heat and oil, fuels and other detrimental fluid exposure. Therefore, because of such heat and fluid exposure, the use of undesirable fluorosilicone materials is mandated therein.
Efforts to mount aircraft engines which include linkages 11a, 11b can be found in U.S. Pat. No. 3,727,862 to Kaufold et al. However, Kaufold provided a fairly rigid vibration path into the structure for vertically oriented vibrations. Moreover, the spring elements are torsion bars 7a, 7b, thus do not provide any significant level of damping. Moreover, the system cannot be easily tuned. Other engine mounting systems include some form of linkage include U.S. Pat. No. 4,717,094 to Chee entitled "Aircraft Engine Mount System With Vibration Isolators," U.S. Pat. No. 3,836,100 to Von Hardenberg et al. entitled "Engine Mounting Arrangement," U.S. Pat. No. 4,805,851 to Herbst entitled "Turbine Engine Mounting Bracket Assembly," and U.S. Pat No. 5.108,045 to Law et al. entitled "Engine Mounting Assembly." The desk reference by A. B. Davey and A. R. Payne entitled "Rubber In Engineering Practice" teaches in FIG. 5.14(a) a focalized suspension system including a plurality of pivotable mounts attached between the engine bracketry and the structure. This system tends to be bulky and the isolators are still subjected to elevated temperatures.
Accordingly, there exists a need for a system for suspending and isolating an engine, such as an APU from the structure, such as an aircraft fuselage which is cost effective, light in weight, easily tuned and exhibits improved service life.